Technologies employed today for the separation of gases include the following: cryogenic fractionation, absorption in a selective solvent, membrane permeation and adsorption on a solid material, such as activated carbon, a molecular sieve or silica gel.
Cryogenic separations require a combination of low temperature and high pressure to liquify the gases so that subsequently they may be separated by distillation. The ability to obtain high purity products is a function of the relative volatility of the components at cryogenic temperatures. Cryogenic processes are generally energy intensive and require high capital investment.
Absorption processes employ a solvent which either physically absorbs or chemically combines with one or more of the gases to be separated. A stripper is required to remove the absorbed material from the solvent. The stripper is usually a fractionation tower and heat is applied to strip the absorbed gases. The lean solvent must then be cooled and recycled back to the absorber which likewise is usually a fractionation tower.
The "SELEXOL" process owned by Norton Company and the "RECTISOL" process developed by Lurgi Gesellschaft fur Warmetechnik are examples of physical absorption processes which use different solvents to remove carbon dioxide and hydrogen sulfide from gas streams. The "COSORB" process developed by Tenneco Chemicals, Inc. is an example of a chemical absorption process. A special solvent can chemically complex and reversibly decomplex carbon monoxide so that the carbon monoxide is preferentially removed from gas streams. Other chemical solvents are monoethanolamine and diethanolamine which are used to recover hydrogen sulfide and carbon dioxide from gas streams. Chemical absorption is very specific and cannot be applied universally for all gas separation processes.
Membrane permeation is based on the fact that when a pressure differential exists across a special membrane, one gas will pass through the membrane more rapidly than others. Monsanto's "PRISM" permeators are presently used to recover hydrogen from ammonia plant purge gas.
Adsorption is a single unit operation which can separate gases with minimum expenditure of energy and capital. The more conventional adsorption processes employ a multi-bed cyclic operation and are thermally regenerated. Examples of this concept are described in my U.S. Pat. No. 3,455,089, issued July 15, 1970, entitled "Process For Removing Organic Contaminants From Air", and in my U.S. Pat. No. 3,534,529, issued Oct. 20, 1970, entitled "Process For Recovering Organic Vapors From Air Streams".
Cyclic, Thermal Swing Adsorbers (TSA) are inherently inefficient and many continuous processes have been proposed to improve their efficiencies. One such process is described in my U.S. Pat. No. 4,231,764, issued Nov. 4, 1980, entitled "System For Removing Organic Contaminants From Air". This process employs a multi-stage fluid bed adsorption system which offers many advantages over multi-bed cyclic systems. However, this process also presents disadvantages such as the high energy cost to maintain fluidization, attrition of the adsorbent, inability to vary gas flow and the limiting gas velocity required to prevent adsorbent entrainment.
Pressure-Swing Adsorption (PSA) is an alternate method for regenerating an adsorbent. As compared to temperature differentials used with TSA, a lower pressure or a vacuum is used to desorb the adsorbent bed. The PSA cycle can be operated close to isothermal conditions without the heating and cooling steps associated with TSA. With PSA, short cycles are possible, thus permitting smaller adsorbers and less adsorbent inventory. For these reasons, the pressure swing cycle is attractive for bulk separation operations as applied in the gas processing industries. One advantage of PSA is the ability to use gas compression as the main source of energy.
The principal disadvantage of PSA is the high gas loss resulting from the pressure release during desorption. This loss can be minimized by employing four or more beds so that depressurization of one bed can be used to purge and repressurize the other beds. This concept has been incorporated in Union Carbide Corporation's "HYSIV" PSA system for hydrogen recovery and purification. The number of beds employed is based on economic considerations; the increase in recovered product must justify the additional capital investment associated with the increased number of beds.
Union Carbide Corporation has developed a single-bed adsorption process using the PSA concept. The system, called Pressure-Swing Parametric Pumping, operates in a single adsorbent bed using the PSA concept with very short and rapid cycles of pressurization and depressurization. Pressure gradients in the bed are developed by the pressure pulses which provide internal purge gas for regeneration, as well as a product flow which is almost continuous. The process is described in Chemical Engineering, Nov. 30, 1981, page 71.
There presently exists a need for continuous adsorption systems which can provide high efficiency and flexibility at minimal costs.